Technical Abstract:
Mortality records from 8,642 lambs from a composite population at the U.S. Meat Animal Research Center were studied using time-to-event Weibull and Cox sire models. To account for the different genetic and environmental sources of variation from birth to 365 d of age (BY), survival from birth to weaning (BW) and from weaning to 365 d of age (WY) were also studied. Records from lambs culled during the study were treated as censored on the day of culling. Male lambs had a significantly (P < 0.01) greater hazard of mortality (23 to 40%) than female lambs in all periods. Type of birth was significant (P < 0.01) in the BW and BY periods with multiple born lambs having a greater hazard of mortality than single- or twin-born lambs. Age of dam was significant (P < 0.01) in the BW and BY periods with lambs from 1-year-old dams having a greater hazard of mortality than those from older dams. Lambs raised by their dams had a significantly (P < 0.01) lower hazard than lambs raised in the nursery in all periods. Estimates o sire variance and heritability from the Weibull model were greater than those from the Cox model in all periods. Modal estimates (and standard errors) of heritability from the Weibull and Cox models in the BW, WY and BY periods were 0.21(0.07) and 0.19(0.12), 0.15(0.05) and 0.16(0.06), 0.18(0.12) and 0.12(0.05), respectively. These estimates indicate potential for effective selection for increased lamb survival, enhancing sheep production and welfare.